In this case, the measuring transducer is located close to the process and decentrally in the field area and is equipped with sensor means for receiving a physical process variable and means for its conversion into an electrical variable.
For a plurality of measuring transducers, means for their energy supply and for measured value processing and visualization are provided in a central waiting area.
In a two-wire measuring transducer, both the electrical energy supply of the active assemblies of the measuring transducer and the transmission of the measured values to the means of measured value processing and visualization in the waiting area are implemented via a single two-wire line between the measuring transducer in the field area and a measuring transducer supply unit located in the waiting area.
For this purpose, starting from a voltage source arranged in the measuring transducer supply unit, a current loop is formed via the first wire of the two-wire line, the internal resistance of the measuring transducer, the second wire of the two-wire line, and a measuring resistor arranged in the measuring transducer supply unit, the loop current of said current loop being determined as a function of the detected process variable by adequately changing the internal resistance of the measuring transducer. The voltage drop at the measuring resistor is then a measure of the detected process variable of the measuring transducer.
A minimum input voltage across the internal resistance of the measuring transducer is required for supplying energy to the active assemblies of the measuring transducer. If the line impedance is too high and/or the output voltage of the measuring transducer supply unit is too low, a high loop current, as a function of the detected process variable, leads to a high voltage drop across the two-wire line, wherein the required minimum input voltage of the measuring transducer statically falls short.
In applications with long line lengths, such as, for example, tank farms or installations with a central switching room with evaluation units and widely distributed measuring points, increased voltage drop can occur along the current loop. The resistance of the connecting line is, furthermore, dependent upon the ambient temperature. Between the contact points (connection and distribution terminals), transition resistances arise, which depend, inter alia, upon the atmospheric humidity and salt content of the ambient air and tend to increase over the life cycle (corrosion). A voltage drop forms across these resistances when current flows and reduces the operating voltage available to the two-wire measuring transducer. In this case, it may happen that the voltage drop along the line and across the contact points becomes so great that the minimum permissible supply voltage of the measuring transducer falls short.
For this reason, measuring transducers in some cases are monitored and assume a safe fault current or are reset and restarted in the event of a shortfall. This detection is used to avoid unsafe operation with an undefined output signal. Particularly critical is the state when, given a small loop current, the permissible minimum supply voltage of the measuring transducer is still maintained, but does not fall short until the loop current increases. A creeping increase in resistance in the current loop then results in failures when the loop current assumes a higher magnitude.
DE 10 2005 047 894 A1 discloses a method for measuring the supply voltage at the current loop connection of the measuring transducer and going into the safe state when an undervoltage is detected.
The disadvantage of this method is the need to periodically pass through the entire current range, independently of the measured values, in order to thus detect shortfalls in the minimum permissible supply voltage. During this measurement, the evaluation unit or process control must be informed that the measured loop current does not correspond to a measurement signal. The measurement thus requires synchronization between maintenance work and process control with respect to the evaluation of the input signals. Monitoring during normal operation is desirable.